The present invention relates to automatic chemical testing apparatus, and more particularly to a method in apparatus for producing signals indicative of the concentration of substances in an analyzed sample.
A suitable environment contemplated for the present invention is disclosed in commonly assigned U.S. Pat. No. 3,728,079 issued Apr. 17, 1973 to John J. Moran, or U.S. Pat. No. 4,043,756 issued Aug. 23, 1977 to David E. Sommervold the disclosures of which are incorporated herein by reference. U.S. Pat. No. 3,622,279 issued Nov. 23, 1971 to John J. Moran, also commonly assigned and having its disclosure incorporated herein by reference, is particularly directed toward means for providing readout signals in response to reacted contents in reaction containers.
In the type of apparatus under consideration, aliquots of a sample generally comprising human serum are each dispensed into a reaction container and reagents are added thereto. After sufficient incubation, a reading is made of the contents of the reaction container to provide a signal indicative of a particular parameter. In the preferred form, the reading is made spectrophotometrically, and an analog signal indicative of optical density is provided. In order to determine concentration of the particular substance of which analysis is being made of the particular aliquot, the optical density reading must be translated into a reading indicative of concentration units of a substance.
A means of providing an output indicative of concentration units is by the use of the electrical circuitry and printout means. The circuitry utilizes known techniques to process the analog readout signal indicative of optical density into an output signal for provision to printing or display means indicative of concentration units. The circuitry employs a linear curve relating optical density to concentration of the substance which has been empirically determined. (Curve fitting techniques are well-known in the art, and do not form a part of the present invention. Therefore, curve fitting techniques and circuitry are not discussed in great detail herein.)
Normally, a single spectrophotometric reading is made in the case of an end point chemistry. This reading is related to its position on the aforementioned linear curve. In the case of a kinetic reaction, one which is characterized by the difference in optical densities at first and second reading times rather than by a single density at a single reading time, one reading is taken at each of the first and second times to define a slope of optical density versus time. While such techniques have provided reliable readings, they rely on a limited amount of measurement. Only one reading is made for one optical density point. In some embodiments, a few readings of one point may be made for assuring reliability. However, in none of these embodiments are the readings taken statistically large. In the present description, statistically large comprehends a number of readings that is large with respect to prior art embodiments. In another form of prior art apparatus, disclosed in U.S. Pat. No. 4,052,161 issued Oct. 4, 1977 to Atwood et al, a measurement period is provided which is extended with respect to other prior art measurement periods as is also contemplated by the present invention. However, a continuous optical absorbence reading is provided which varies to vary the input to a voltage to frequency converter having an output connected to counters. In that prior art embodiment, an optical absorbence curve from which concentration units in the sample may be determined is generated. However, a plurality of individual optical density values are not generated.